1. Field of the Invention
This invention relates to an improvement in a frequency inverter which translates an input alternating current power to an alternating current power of a higher frequency.
2. Description of the Prior Art
One of the basic structures of a known frequency inverter is shown in FIG. 1. An alternating current from an alternating power source 1 is rectified by a full-wave rectifier 2 into a DC current, which is fed across bus lines 3 and 4, the DC current is smoothed by a condenser 13, and a smoothed electric power is fed to a positive bus line 6 of a high-frequency part through a choking coil 5 from the positive bus line 3. Between the bus lines 4 and 6, a thyristor 7 and a return diode 8 are connected in parallel with and reversely to each other, and a series resonant circuit consisting of a condenser 9 and an output coil 10, which form a turning circuit of the thyristor 7, is also connected in parallel to the thyristor 7 and the return diode 8. Driving of the thyristor 7 is carried out by a gate-trigger circuit 12, and a start and stop of a high-frequency-chopping operation of the gate-trigger circuit 12 are controlled by a start/stop-control circuit 11. An application of reverse-bias supression voltage to the gate terminal of the thyristor 7 is generally used for shortening turning-off time of the thyristor. We explain on a typical convertional circuit as follows.
A principal part of a specific circuit of the conventional art illustrated in FIG. 1 is shown in FIG. 2, and voltage and current wave forms of various parts are shown in FIG. 3. The gate-trigger circuit 12 of FIG. 2 comprises an astable multivibrator 15 and transistors as amplifier. Wave forms of voltages between the collector-emitter electrodes (they are referred to as "VCE" hereinafter) of the output transistors Tr1, Tr2 of the astable multivibrator 15 are shown in FIG. 3 (c) and (d) respectively. The triggering voltage of the thyristor 7 is obtained by a pulse obtained by differentiating the output voltage V.sub.CE of the transistor Tr2. Namely on rising of the voltage V.sub.CE of the transistor Tr2, a transistor Tr3 turns on by a pulse made by a differential circuit consisting of a condenser 16 and a resistor 14, and accordingly a transistor Tr4 turns on. On the other hand, a zener voltage of a zener diode 17, connected to the collector electrode of the transistor Tr1, is selected larger than a negative voltage (-E2) and smaller than a sum of positive voltage (E1) and negative voltage in advance. As a result, the zener diode 17 turns into a break-over state when the transistor Tr1 turns off, and thereby makes the transistor Tr5 on. The output voltages of the transistor Tr4 and Tr5 are fed respectively as a triggering voltage and a reverse-bias voltage through a terminal G (see FIG. 3. (e)). Therefore the thyristor 7 turns on by the triggering voltage and a positive current illustrated in FIG. 3 (a) flows in the thyristor 7 in the direction from the output coil 10 to the condenser 9. A negative current shown in this figure FIG. 3 (a) is a return current flowing in the return diode 8 in the direction from the condenser 9 to the output coil 10. The time period during which the reverse current flows is a margin time for a turning of the thyristor 7. When this current extinguishes, a forward voltage is applied across the anode and cathode electrodes shown in FIG. 3 (b). In general, the application of the reverse-bias voltage of the thyristor is effective only when the return current is flowing (for preventing the tuning failure of the thyristor) and when forward edge of application of the forward-direction voltage (for preventing dv/dt firing of the thyristor). And applications of reverse-bias at other occasions than the above-mentioned cases are in vain.
However, in this conventional apparatus, the application of the reverse-bias lasts untill just before the next triggering voltage, and accordingly, the reverse-voltage is uselessly impressed during an unnecessary time period. Therefore, this conventional apparatus has the following disadvantages.
(1) Power loss is large, and
(2) There is a danger that either one or both transistors Tr4 and Tr5 may be destroyed, since there is a possibility that too much current flows through the transistors Tr4 and Tr5 when both become conductive because of charge accumulation times of the transistors. And simultaneously a rising up wave form of the triggering voltage of the thyristor 7 may become dull by the charge accumulation, and therefore a switching loss of the thyristor 7 becomes large. In order to solve these problems, there is another proposal that the transistor Tr5 is made on only for a short time by a signal made by differentiating the output signal of the transistor Tr1, thereby to apply the reverse-bias voltage to the gate of the thyristor 7 for that short time. But in such case, there are still the following problems.
(1) A circuit structure becomes complex, and
(2) The timing of the gate signals does not synchronize to the returning current from the load to the diode 8 when condition of the load changes.
There is another prior art example which uses a pulse transformer PT. The circuit of a principal part of an example of such conventional apparatus is shown in FIG. 4. A transistor Tr6 is driven by the output signals of the transistor Tr2 of the same astable multivibrator 15 as the aforementioned prior apparatus of FIGS. 1 and 2. A primary coil of a pulse transformer PT is excited by the collector current of the transistor Tr6 until the transistor Tr2 becomes on, and thereby a positive voltage is generated in the secondary coil of the pulse transformer PT. Upon turning on of the transistor Tr2, a reverse voltage is generated in the secondary coil of the pulse transformer PT. The positive and negative output signals are applied to the gate terminal G of the thyristor 7 as a trigger voltage and a reverse-bias voltage, respectively as shown in FIG. 5. In the example of FIG. 4, though its circuit is simple, there are the following problems:
(1) Power loss is large, since the triggering voltage is applied for unnecessarily long time.
(2) The reverse voltage is generated by the electromagnetic energy stored at the inductance of the pulse transformer PT, and therefore, the reverse-bias application time is limited to a short time.
(3) The reverse-bias voltage is under the influence of the gate impedance of the thyristor.